In the manufacture of VLSI semiconductor devices, multilevel interconnects are made to increase the packing density of devices on a wafer. This also requires multilevel dielectric layers to be deposited between conductive layers. Such dielectric layers must have good step coverage and planarization properties to produce void-free layers that not only completely fill steps and openings in the underlying substrate, but also form smooth, planarized dielectric layers. Further, such dielectric layers must be able to be deposited at low temperatures, preferably below about 400.degree. C., to avoid damage to underlying, already formed interconnects.
It is known that silicon oxide dielectric layers can be deposited with good conformality and planarization using tetraethoxysilane (hereinafter TEOS), ozone and oxygen at comparatively low temperatures, e.g., about 375.degree. C. It is also known that the ratio of TEOS and ozone affects the film quality and deposition rate. For example, when depositing silicon oxide from TEOS and ozone using a high ozone:TEOS flow ratio, the rate of deposition is reduced, but the film quality is higher and conformality, i.e., the ability to produce void-free filling and planarized layers, is also higher. The layers provide good step coverage and good conformality which leads to excellent planarization, and excellent quality of the silicon oxide when deposited onto silicon. However, when the silicon oxide is deposited onto silicon oxide, for example a thermally grown silicon oxide layer, differences in film quality have arisen. The deposition rate of low temperature silicon oxide films from TEOS and ozone deposited onto thermal silicon oxide is about 20% lower, and the wet etch rate of silicon oxide is high, which is indicative of poor quality films. Furthermore, the surface of the silicon oxide is very rough and contains voids, indicative of a porous film.
Fujino et al have addressed this problem. Their solution is a two step deposition process; first a deposit of silicon oxide using a low ozone concentration (0.5%) is put down, and a second layer deposited thereover using a high ozone concentration (5%).The result is said to be improved film quality of silicon oxide planarizing films.
However, this ozone-TEOS process still has limitations in terms of poor surface quality and surface sensitivity. The higher the ozone:TEOS ratio, the greater the decrease in oxide deposition rate, and the greater the degradation in film properties, such as wet etch rate, water resistance and stress drift with time.
Thus an improved process for depositing silicon oxide layers from TEOS and ozone having excellent conformality and film quality is desirable, particularly for the manufacture of VLSI devices.